FIELD OF THE INVENTION
The invention relates to a method for compensating for propagation time delays in a communication network having a plurality of subscribers which are each connected to a ring feeder for the common transmission of digital source data and control data in a data stream synchronous with a clock signal, the source data and control data are transmitted in a format which prescribes a pulsed sequence of individual bit groups of identical length that contain component bit groups which in each case form a data channel for source data or control data, and at least two subscribers, which are connected at different locations to the ring feeder, receive source data that are transmitted by one of the subscribers on different data channels and are correlated with one another. The invention also relates to such a method used in a mobile communication system, in particular a communication system in a motor vehicle.
Such an annular communication network as is described, for example, in Published, European Patent Application 0 725 522 A1 serves for networking different types of electric and electronic devices which are intended to exchange information among one another in a partly complicated way, with the aid of data lines of physically simple construction. The subscribers can exchange both source data and control data through such data lines. For example, in the audio field it is possible to transmit audio data from data sources such as CD players, radio receivers and cassette recorders to data sinks such as amplifier/loudspeaker combinations, and control data can be transmitted at the same time, for example in order to control volume. In such a case, a device can be constructed simultaneously as a data source and a data sink, such as is the case with a cassette recorder, for example.
A certain delay is necessarily produced in each subscriber when reading out the source data and control data from the ring feeder, subjecting them to intermediate repetition and processing and reading them into the ring feeder again. That signal delay is particularly problematic when a subscriber transmits different source data which are mutually correlated in time and are received by different subscribers that are connected at different locations on the ring feeder. That is the case, for example, for devices in entertainment electronics which supply at least two amplifier/loudspeaker combinations with audio signals on a plurality of separate channels, two in the case of stereophony.
Although the delay in each subscriber can be restricted technically to a few tens of microseconds, in the case of a plurality of loudspeakers distributed in the network, which are separated by a plurality of other subscribers, the signal delays in the individual subscribers can add up to a magnitude which can cause a loss of the stereo impression or other spatial impressions, or even give rise to an unpleasant auditory impression. However, even if the signal delay is so small that no impairment of the stereo effect is perceived, more recent methods for generating three-dimensional auditory impressions such as, for example, pseudo stereophony would be ruled out.
In a communication network of the type described above it is complicated to compensate for the signal delays which occur. A particular advantage of such a network resides specifically in its flexibility with regard to cable laying, and the way in which further subscribers can be inserted as desired without changing the circuitry of the remaining system. In order not to lose that advantage, the positions of the current data source and the data sinks associated therewith have previously been determined in top protocol layers of the communication controller, something which has required a multiplicity of messages to be exchanged between subscribers with the aid of control data. The number of subscribers between the data source and the data sinks or between the individual data sinks can be calculated from the positions determined in that way.
Since the signal delay is known for each individual subscriber, it is then possible to calculate corresponding forced delays with which the mutually correlated signals can be correctly reproduced. Those delays can be subsequently reduced in any known way, for example by brief intermediate storage of the source data in the data sinks.